Television receivers that include an AC power line input transformer are generally desireable because, inter alia, they provide a substantial degree of isolation between the AC line return and the television receiver ground. However, because of their size, weight and cost, power line transformers are often necessarily omitted in the design of certain types of television receivers; for example, portable models such as those usually found in hotels and motels will often be of the "hot chassis" variety, that is, the receiver ground will be connected to the AC line return.
It has been found that one of the performance problems posed by hot chassis receivers, especially when used in conjunction with MATV or CATV systems, is that of direct pickup. That is, if the receiver is operating in an area where there is relatively strong ambient field, such as may be generated by a nearby TV transmitter, then the outer shield of the coaxial cable carrying the MATV or CATV signal will pick up a voltage as a result of that field. Unless precautions are taken, this voltage will be coupled to the input of the TV receiver. Specifically, a CATV system close to a TV transmitter broadcasting the same channel as that being carried on the cable will produce a second image on the TV screen displaced in position from the image produced by the desired signal.
A number of approaches have been undertaken in an effort to eliminate the direct pickup problem. Some receivers have resorted to the use of back-to-back balun (balanced-to-unbalanced) transformers. However, the degree of immunity to direct pickup is directly related to the balance ratio of the back to back baluns and may be inadequate in an environment characterized by strong ambient fields.
Another approach is to use two "capristors" one connected between the coaxial cable center conductor and the receiver input and the other connected between the coaxial shield and receiver ground. However, the degree of immunity to the direct pickup voltage is inversely related to the impedance between the cable shield and the TV receiver ground, that is, to the effective impedance of the capristor at the operating frequency. Because of the capristors lead inductance and equivalent series resistance, this impedance is often greater than the impedance necessary to provide the desired immunity and some amount of direct pickup voltage will be coupled to the input of the receiver.
This invention is directed to a TV isolation system for a hot chassis receiver that provides improved immunity to direct pickup.